The present invention relates to a device for amplifying the intensity of an electro-optically created image.
One of the earliest devices of this type, generally referred to as optical image intensifiers, is described for example in Swiss Pat. No. 301,222. The main operating principle of this device consists of imaging a system of bars, arranged parallel to and spaced apart from one another, on a related second bar system, by means of a reflecting zone. The reflecting zone is located on a control layer which can be deformed by electrostatic field forces and is arranged, together with a photoelectric conducting layer, in an electrostatic field. The picture to be amplified is imaged as a grid-like pattern on the photo-electric conducting layer, thereby locally changing the electrostatic field in accordance with the image and effecting a corresponding deformation of the control layer and the reflecting zone. The deformed reflecting zone is imaged between the bars of a second bar system onto a projection screen on which an image corresponding to the image to be amplified, but of greater brightness, is visible.
To produce an electrostatic field according to the device disclosed in Swiss Pat. No. 301,222, two transparent homogenous electrode surfaces are connected to one of the photoelectric conducting layer or the control layer and are linked to a power source. During exposure of the photoelectric conducting layer to the image, its electrical resistance changes. The resistance change in the longitudinal direction, that is parallel to the electrical field, causes a local change of the voltage distribution between the photoelectric conducting layer and the interspace between the conducting layer and the control layer, which in turn causes a corresponding deformation of the control layer and the reflecting zone.
This operating principle, which utilizes the resistance change of the photoelectric conducting layer in the longitudinal direction, has several drawbacks which are explained, for example, in detail in Swiss Pat. No. 378,432. Among other drawbacks, a relatively large minimal thickness of the photoelectric conducting layer is required to obtain useful control. However, this poses the potential problem of disturbing spacecharge effects.
In Swiss Pat. No. 378,432, an optical image intensifier is described in which the drawbacks of the previously described image intensifier are avoided by using transverse local electrical conductance or resistance changes rather than longitudinal changes. The electrode which is in contact with the photoelectric conducting layer is formed as a strip grid wherein the electrically conducting strips which are arranged with regular spacing next to one another are disposed at right angles to the bars of the first bar system and are connected in alternating sequence to one or the other pole of the electrical power source. In addition, an optical strip grid is placed in front of the electrode grid on the exposure side; which comprises strips that are preferably disposed so that they form an angle of 45.degree. with the electrode grid.
With this electrode configuration and arrangement, the potential distribution in the photoelectric conducting layer changes during exposure and it is this potential distribution in the layer alone which determines the forces acting on the control layer or the reflecting zone. The photoelectric conducting layer therefore can be as thin as desired as long as its electrical resistance is not impaired by leakage through its base. The difficulty of high specific electrical resistance modulation is also eliminated because a suitable selection of the voltage between the electrode strips and the thickness of the layer allows for every practically usable value to be utilized.
Despite these evident improvements versus the image intensifiers that operate with longitudinal modulation, the image intensifier described in Swiss Pat. No. 378,432 did not gain general acceptance in practice. One of the principal reasons for this fact is the undesired base deformation of the control layer that occurs with the alternating polarity of the individual electrode strips particular to this image intensifier, which is much larger than the intended useful deformation caused by exposure to the image. This unfavorable relationship between the base and useful deformation is equivalent to low sensitivity and small efficiency of the entire image intensifying arrangement. A further difficulty associated with this image intensifier is caused by practically unavoidable local short circuits between neighboring strips of the electrode grid. When a short circuit occurs in the intensifier, it does not cause just a local disturbance of the image, but rather a total loss of the whole picture. The image intensifier completely loses its capacity to function in case of a short circuit.
It is therefore an object of the present invention to provide a novel optical image intensifier having operating characteristics that are substantially improved over those of a device of the type described in Swiss Pat. No. 378,432, with respect to efficiency and short circuit sensitivity.